1. Field of the Invention
The present invention relates to a lock mechanism for a webbing retractor employed in a seatbelt system designed to protect an occupant of a vehicle when an emergency situation occurs, the lock mechanism being adapted to prevent an occupant retraining webbing from being wound off at the time of such an emergency.
2. Description of the Prior Art
A typical seatbelt system includes a webbing retractor into which an occupant restraining webbing is wound up from one end thereof by means of a predetermined biasing force and which incorporates a lock mechanism adapted to prevent the webbing from being wound off when a vehicular emergency occurs.
The lock mechanism has a lock ring provided in such a manner that the lock ring rotates together with the rotation of a webbing takeup shaft in the webbing wind-off direction and a resilient member interposed between the lock ring and the takeup shaft. Thus, when a vehicular emergency occurs, an acceleration sensing means causes the lock ring to be subjected to resistance which acts against the rotation of the lock ring, and a lock member which rotates together with the takeup shaft is therefore driven with a lag in terms of rotation of the lock ring relative to the takeup shaft and is engaged with lock teeth formed on a frame (see specifications of Japanese Patent Publication No. 16,969/1969 and Japanese Patent Laid-Open No. 1,115/1972).
This type of conventional lock mechanism, however, suffers from the following problems. Namely, when a vehicle is running on a rough road with many irregularities, the acceleration sensing means may be undesirably kept in an operative state. If so, the lock mechanism is unfavorably maintained in its locked state, which fact makes it impossible for the occupant to wind off the webbing. Further, during such running on a rough road, the occupant is repeatedly bumped up and down by the vibration of the vehicle, and the webbing is thereby repeatedly wound in and out. As a result, every time the webbing is wound in, the position of engagement between the lock member and the lock teeth gradually advances, so that the webbing is gradually wound up into the webbing retractor, which fact may unfavorably increase the pressure applied to the body of the occupant.